Sodium alginate and calcium carbonate are natural materials with a high compositional variability. The molar mass and structural composition of sodium alginate and the crystalline structure of calcium carbonate can affect their interactions in pure water. Herein, we studied the adsorption of sodium alginate onto calcium carbonate microparticles in pure water. The adsorption appears to be driven by electrostatic interactions stabilized by calcium ions at the surface of the particles and in solution due to the dissolution of calcium carbonate in pure water. We found that the adsorption of sodium alginate is favored onto microparticles with more calcite. We also found that the adsorption of sodium alginate polymers with low guluronate content (∼ 30%) onto calcium carbonate particles tends to increase for polymers with lower average molar mass and more flexible polymer chains. Polymers with high guluronate content (∼ 65%), exhibiting a more rigid conformation, tend to adsorb more onto calcite when they present a lower average molar mass polymers as well as a higher proportion of guluronate monomers.